This invention relates to a fluidized bed heat exchanger and, more particularly, to a heat exchanger in which heat is generated by the combustion of particulate fuel in a fluidized bed and a method of operation thereof.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a normal fluidized bed arrangement, air is passed through a perforated plate or grid supporting particulate material which usually includes a mixture of a fuel material, such as high sulfur bituminous coal, and an adsorbent material for adsorbing the sulfur released as a result of the combustion of the coal. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, substantially uniform bed temperature, combustion at relatively low temperatures, ease of handling the coal, a reduction in corrosion and boiler fouling and a reduction in boiler size.
In the fluidized bed combustion process, the coal and adsorbent are continuously introduced into the bed by suitable feeders, injectors, or the like, and coal ash and adsorbent are discharged from the lower portion of the bed, usually through a gravity drain pipe having an entrance registering with a discharge opening formed through the perforated support plate and a distal end communicating with a screw cooler, a conveyor belt, or the like. However, in arrangements in which the size of the coal extends over a relatively large range, relatively heavy pieces of coal ash tend to migrate to an area above the drain pipe and form a dense area that is difficult, if not impossible, to fluidize. As a result, the heavy pieces of coal ash do not drain, but rather cause a clogging of the drain pipe and an attendant severe curtailment in the operating efficiency of the bed.
Furthermore, in the operation of the fluidized bed, in order to maximize heat transfer efficiency, it is desirable to maintain close control over the level of material in the bed. Precise control is difficult to achieve in a fluidized bed in which new material is continuously being introduced, if the drain tends to become clogged. Moreover, it is desirable to not only maintain a continuously controllable discharge through the drain, but it is also desirable to retain the relatively light adsorbent material particles in the fluidized bed, while permitting only the relatively heavy coal ash particles to discharge through the drain. In this manner, the adsorbent material is retained in the fluidized bed for a longer time to adsorb more sulfur from the combustion of the coal and, as a result, less new adsorbent material need be continuously introduced. There is an acceptable loss or attrition of adsorbent material in the normal operation of the bed by the reduction of the adsorbent material to fine particle size due to the boiling action of the bed and the grinding of the particles against one another, and by the entrainment of the fine adsorbent material particles in the fluidizing gas, by which they are carried out through the flue.